The dispossessed and invisible man Darko Suvin defines science fiction as "a literary genre whose necessary and sufficient conditions are the presence and interaction of estrangement and cognition, and whose main formal device" (Suvin 7 -8) is a radically or at least significantly alternative "locus and/or dramatis personae". the author "simultaneously perceived as not impossible within the cognitive norms (cosmological and anthropological) of the author's time" (Suvin viii). Unlike fantasy, science fiction is set in a realistic, yet strange, alien world. Except that there are limits to how alien another world, another culture, can be, and it's the interface between these two realms that can give science fiction its power, making us look back at ourselves from its distorted perspective. novum a general theory of time, illustrated by the paradox of a stone thrown against a tree, a stone that can never reach its target because "there is always half the way to go" (Le Guin 26). Shevek, Le Guin's protagonist and formulator of general temporal theory, sees himself as one who "'takes down walls'" (Le Guin 289), as the "primordial number, which [is] both unity and plurality" (Le Guin 30) cross interfaces. Walls abound in The Dispossessed: the wall between Anarres and Urras (Le Guin 1-2), the wall that separates one individual from every other (Le Guin 6), the wall of social consciousness (Le Guin 287), the wall between men and women (Le Guin 14-16), the wall of time - Zeno's paradox - the limit that prevents the rock from hitting the tree (Le Guin 26). But as Shevek knows, the rock hits the tree; that's the joke (Le Guin 27). The wall can be crossed. Crosses it leaving Anarres; he crosses it in his love for Takver and Sadik; he crosses him with the Mayor of the Initiative, and he crosses him with the Terrans and the Hainese. This need to "break down walls" is its "cellular function", its "moral choice", but it is a "process" and not an "end", a "journey and return" and not simply a "repetitive, timeless" cycle . (Le Guin 290-291). The paradox of sequence and simultaneity is that nothing remains the same; it is not the same river passing along the bank, or the same wind blowing through the same tree last spring.